Fixed #21874 -- Require Django applications to have a filesystem path.

Wherever possible this filesystem path is derived automatically from the app
module's ``__path__`` and ``__file__`` attributes (this avoids any
backwards-compatibility problems).

AppConfig allows specifying an app's filesystem location explicitly, which
overrides all autodetection based on ``__path__`` and ``__file__``. This
permits Django to support any type of module as an app (namespace packages,
fake modules, modules loaded by other hypothetical non-filesystem module
loaders), as long as the app is configured with an explicit filesystem path.

Thanks Aymeric for review and discussion.
Fixed #21853 -- Fixed Manager.__module__ to properly return 'django.db.models.manager'.

The combination of BaseManager.from_queryset() and RenameMethodsBase results in
Manager.__module__ having the wrong value. This can be an issue when trying to
pickle the Manager class.
Fixed #21849 -- Included the count of silenced system checks in output.
Add an extra warning about custom save methods in migrations docs.
Fixed #21883 -- Added six.moves.xrange import

When moving code around from sql.where to lookups six.moves.xrange
import was forgotten. The xrange import is needed on Python 3.
Added tests for m2m queries with custom pk on the end models

It seems this case was fixed somewhere between 1.5.x and 1.6.x. I added
tests as I wasn't able to find any tests for these cases. Refs #21879
Made Oracle introspect boolean fields

Fixed failing test schema.tests.SchemaTests.test_add_field_temp_default_boolean
Refs #19884
Fix typo CRSF -> CSRF
Merge pull request #2224 from Ian-Foote/patch-1

Fix typo CRSF -> CSRF
Made Oracle introspect FloatFields correctly

Broke InspectDBTestCase.test_field_types in two:
- a test_number_field_types, which now passes on Oracle too
- a test_field_types, for all non-numeric fields, which is still expected to fail

Also made some pep8 fixes in the tests file. Refs #19884

Thanks Tim Graham for review.
Added new srs test and various cleaning
Fixed #21907 -- Fixed add_srs_entry for Spatialite >= 4

Thanks dfunckt for the report.